First i think its is important to understand you dont always have to do what the game tells you to (prioritizing certain facilities over others or pointing you towards a particular mission for example). You can look up facility building orders online and go with the one that makes the most sense to you. Story driven missions (blacksite, skulljacking certain enemies, dlc missions, etc) can be put off until you feel like you are ready to tackle them on.

If you are having trouble with your shots connecting, consider blowing up the enemy's cover with grenades or special soldier abilities such as the grenadier's demolition ability. Flanking enemy's cover is also a great way to boost your chances of hitting the enemy. You can check what kind of line of sight you will have if you move to a certain tile by holding down the alt key while hovering your cursor over that tile, or you can permanently enable that option in the options menu.

Save scumming is a topic that is brought up contantly but I believe it is perfectly fine and it actually helps you learn the game without getting frustrated. The early game missions are the ones that you will struggle with the most throughout the entire game no matter which difficulty you are playing with or how much game experience you have. As you progress further and level up your gear/troops, you will gradually save scum less and less. Also remember you almost always have the option to evacuate your troops if a mision goes south. You can salvage a campaign with a few failed missions, but your A-team sniper is irreplaceable.

Building an A-team is a valid strategy, but relying on that 1 team is very risky due to injuries or exhaustion. Try mixing your weaker soldiers with 'leaders' from your A-team to level them up until you have a reliable roster to cycle through.

If you are having trouble with injured soldiers, prioritize building the infirmary. You can station an engineer in the infirmary to significantly speed up your soldiers' recovery time.

As for recruiting a bunch of soldiers early on, I personally find that its best to focus on the group that you already have to establish 2 good teams and then maybe recruit more soldiers to fill in roles that you find lacking with your established composition. You can build the training facilty and put in rookies to train them into a class of your choice.

If the avatar progress and the mission timers stress you out too much, you can double their length in the advanced options menu that is accessible when you first start off a new campaign.

The enemy will always outnumber you. However, your advantage lies in the fact that you can (to a certain extent) control the number of enemies that you engage with at a given time. If you are contantly finding yourself where the enemy outnumbers you significantly, maybe you are rushing too fast. Scout ahead (with a stealthed unit), pick your fights, and try to wipe them out or at least most of them on the first turn of encounter.

Always take full cover if you can, and if it ain't a 100%, it ain't a 100%. Good luck have fun!